Seismic surveying is a standard tool for the exploration of hydrocarbon reservoirs. Vertical seismic profiling (VSP) is one method employed in the art of seismic surveying. VSP can be used within a single well or can be used in multiple wells, i.e., in a cross-well arrangement, which are well known techniques. VSP uses a plurality of sensors arranged within the well. Various types of acoustic and/or pressure sensors known in the art are used in seismology. A seismic generator arranged at the surface or in another well transmits waves, which are reflected by the geologic formations or transmitted through them. The sensors then receive these waves.
It is generally preferred to permanently position the sensors within the well, and further preferred that such sensing not substantially interfere with normal production operation of the well. Various techniques exist in the art to mechanically couple sensors to a borehole structure, such as the production tube, the well casing, or a production packer. In the art, the sensors are typically arranged outside the casing and are surrounded by cement injected into the annular space between the casing and the borehole of the well. Embedding the sensors in this manner is beneficial in that acoustic waves used in the seismic analysis can easily travel to the sensors without attenuation. In addition, different types of acoustic waves (e.g., shear waves) can be sensed using this method. Unfortunately, mechanically coupling the sensors to the casing can be generally difficult and costly to perform. Furthermore, the sensors are not recoverable.
According to other approaches of vertical seismology in the art, sensors are only temporarily located within the well. During temporary placement, the sensors are used to take readings and then retrieved from the well. In addition, the position of the sensors can be changed within the well to take into account alterations of the earth strata under analysis, resulting from production of effluents. Moreover, deployment or retrieval of temporary sensors disrupts production from the well, which can be particularly costly if measurements are periodically made to assess strata conditions over a given time period. Furthermore, preparing the sensors for insertion into the well, properly positioning the sensors, and retrieving the sensors can require tedious preparation and execution.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.